Background KRAS and BRAF V600E mutations are important predictive and prognostic markers, respectively, in colon cancer, but little is known about patient and clinical factors associated with them. Methods Two thousand three hundred twenty-six of 3397 patients in the N0147 phase III adjuvant trial for stage III colon cancer completed a patient questionnaire. Primary tumors were assessed for KRAS and BRAF V600E mutations and defective mismatch repair (dMMR) status. Logistic regression models and categorical data analysis were used to identify associations of patient and tumor characteristics with mutation status. All statistical tests were two-sided. Results KRAS (35%) and BRAF V600E (14%) mutations were nearly mutually exclusive. KRAS mutations were more likely to be present in patients without a family history of colon cancer and never smokers. Tumors with KRAS mutations were less likely to have dMMR (odds ratio [OR] = 0.21; 95% confidence interval [CI] = 0.15 to 0.31; P